It is known to provide wireless data communications by using visible light (or infrared or ultraviolet light) instead of radio frequencies to transmit and receive data wirelessly between devices. Data may be transmitted using visible light by modulating an intensity of the light. The light used may be coherent or incoherent. Wireless networks using visible light may in some circumstances allow a higher data capacity and greater energy efficiency than radio frequency wireless networks, and may also be used to replace point-to-point infrastructure in locations where conventional infrastructure does not exist or is too expensive to build.
The IEEE 802.11 standard is a standard for communication over wireless networks. A wireless network may comprise an access point (AP) and several stations (STA) which each communicate with the access point. To allow multiple stations to communicate with a single access point, the IEEE 802.11 standard provides a distributed coordinated function (DCF) multiple access method and a point coordinated function (PCF) multiple access method.
In the DCF method, a station wishing to transmit data on a channel waits until the channel is not busy before sending the data. A station may transmit data to the access point on a channel once it detects that the channel is not busy for a given time interval. If the data sent by the station is successfully received, the access point sends an acknowledgement (ACK) to the station. If there are several stations, additional measures may be taken to avoid collisions occurring due to all the stations trying to send data at once when the channel ceases to be busy.
In the PCF method, the access point polls the stations sequentially. The access point may perform repeated polling cycles. In each polling cycle it may poll each of the stations in order. A given station is permitted to send data to the access point only in response to that station being polled. If a station does not have data to send, it must nevertheless respond to the poll. When an access point receives uplink data from a station it sends an ACK to acknowledge the receipt of the uplink data. When a station receives downlink data from the access point it sends an ACK to acknowledge the receipt of the downlink data.
The polling of a station in known PCF methods comprises the sending of a poll packet. The poll packet includes an address of the station to be polled and a flag that indicates that the station is being polled. The poll packet includes a MAC frame comprising a MAC header.
The 802.11 PCF method addresses a device by the MAC address of that device. The MAC address is contained in the 802.11 MAC header in accordance with the 802.11 standard (see, for example IEEE Std 802.11-2012, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications). In a poll packet in accordance with the 802.11 standard, the frame control field of the MAC frame has the value for CF-Poll.
The response to a poll message by the station being polled comprises sending a response packet to the access point. The response packet sent from the polled station may also include additional payload data or ACK messages, or may be a minimal response that does not include such additional payload data or ACK messages.
The PCF method of the 802.11 standard has to follow a certain timing because it is designed to coexist with the standard access method DCF. The 802.11 PCF method must allow time intervals in which devices can transmit using DCF.
FIG. 1 is representative of a polling cycle on a simple network using the 802.11 polling scheme. The network includes one access point (AP) and two stations (STA1, STA2). A timeline 10 is shown, with time going from left to right.
Blocks above the timeline 10 represent downlink packets sent by the AP. Blocks below the timeline 10 represent uplink packets sent by stations.
At block 12, the AP polls the first station, STA1, by sending a poll packet addressed to STA1. The poll packet is received and decoded at STA1. Once the complete poll packet has been received and decoded, STA1 sends a response packet (represented by block 14) to the AP. The response packet is received and decoded by the AP.
The decoding of a packet refers to the extraction of data from the packet, and may be used to refer to or comprise demodulation techniques used to extract data from received signals.
At block 16 (which starts after the response packet of block 14 has been received and decoded), the AP polls the second station, STA2, by sending a poll packet addressed to STA2. The poll packet addressed to STA1 is received and decoded by STA2.
At block 18, after the poll packet addressed to STA2 has been received and decoded by STA2, STA2 sends a response packet to the AP. In this example, the response packet sent by STA2 at block 18 also carries a data payload.
Poll packets and response packets may carry payload data and acknowledges (ACKs) if there is anything to send. If the response packet carries a payload of data from a station, a subsequent poll packet addressing that station may include an ACK acknowledging the receipt of that payload data. If payload data is carried in the poll packet, a response to that poll packet may comprise an ACK acknowledging that payload data.